1. Field of Invention
The invention relates to a noise-removing device for removing high-frequency components flowing through a line by grounding a middle point of a DC bus line through a capacitor. More particularly, the invention relates to a noise-removing device adapted to an inverter for controlling electric motors in a drive unit for electric cars and in a drive unit for hybrid cars.
2. Description of Related Art
An inverter for driving electric motors is required by AC electric motors (in this specification, a motor and a generator that also works as a motor are both called electric motors) that are driven by a DC power source, such as a storage battery, as in a drive unit for electric cars and in a drive unit for hybrid cars. In a drive unit mounting, for example, two electric motors, as a motor and a generator, the inverter for use in the drive unit has a circuit structure as illustrated in FIG. 5 (omitting, however, the signal circuit). As shown, the inverter comprises a DC smoothing circuit F connected to a battery B which is a DC power source mounted on a vehicle, and a three-phase AC converter circuit E connected in parallel therewith and connected to the windings of U-, V-, W-phases of the motor M and the generator G. A separately arranged electric motor control unit is controlled based on the control signals output to a signal circuit of the inverter. That is, during power running, a DC current supplied from the battery B through the DC power line S is converted into currents of the U-, V-, W-phases, and these currents are fed to three-phase coils of the electric motors M, G through three-phase AC power lines A. At the time of generation or regeneration, further, the currents of the U-, V-, W-phases generated in the three-phase coils of the electric motors M, G are fed through the three-phase AC power lines A, are converted into DC currents, and are supplied to the battery B through the DC power line S.
In the circuit comprising the inverter, the smoothing circuit F connected to the DC power line S includes a smoothing capacitor C for removing low-frequency components, a parallel resistor R, as well as so-called Y-capacitors which constitute noise-removing means for removing high-frequency components (usually, several hundreds of kilohertz to several megahertz) leaking into the smoothing circuit F by grounding the middle point through a pair of Y-capacitors. Use of the Y-capacitors as a noise filter has been disclosed in JP-A-2000-315929.